The present invention relates to analog optical transmission systems, and more particularly to an apparatus for canceling distortion components introduced by a laser transmitter.
Optical transmission systems are currently being implemented for use in various communication applications. For example, telephone systems are now in use that utilize fiber optic technology to transmit voice and data signals over long distances. Similarly, cable television networks are now available wherein fiber optic technology is used for the transmission of both analog and digital signals.
Prior to the implementation of optical transmission networks, cable television programs were carried as radio frequency (RF) signals over electrical coaxial cables. In fiber optic transmission systems, communication lasers are used for the transmission of the multichannel television signals. The RF signals are used to modulate a light source, and the modulated light is transmitted along the length of an optical fiber.
Optical transmission systems provide substantial benefits, including a virtually unlimited bandwidth and improved system performance. However, harmonic distortion is a major limitation in analog amplitude modulated optical communication transmission systems. Such distortion, and particularly second order distortion, is introduced by the laser which transmits signals over the optical communication system.
U.S. Pat. No. 4,393,518 issued on Jul. 12, 1983 for "Optical Communication Arrangement" discloses an optical transmission system wherein a transmitter transmits an electrical analog input signal as two separate optical signals through a pair of optical fibers to a receiver. One of the optical signals represents the positive portions of the input signal. The other represents the negative portions of the input signal. By combining the two optical signals at the receiver, an electrical analog output signal is generated representative of the input signal, absent even harmonic frequency components that may have been introduced during transmission. The receiver requires various amplifiers including a difference amplifier in order to reconstitute the input signal.
Commonly assigned, copending U.S. patent application Ser. No. 07/436,614 entitled "Distortion Canceling Fiber Optic Transmission System" discloses a scheme in which an analog input signal is communicated to a receiver over one fiber and the inverse of the input signal is communicated to the receiver via a separate, parallel fiber. At the receiver, the signals are combined to cancel harmonic distortion introduced during the signal transmission. Since the entire input signal is transmitted over both of the parallel fibers, the information transmitted can be recovered, albeit at a reduced quality, if one of the fibers breaks.
Both of the aforementioned distortion cancellation schemes have drawbacks. For example, the use of parallel fibers is required, which results in increased expense in the construction of the distribution network. In addition, the design of the receiver is somewhat complex and relatively expensive to implement.
It would be advantageous to provide an improved system for reducing distortion in a fiber optic system that carries analog signals, such as amplitude modulated vestigial sideband (AM-VSB) television signals. The present invention provides such apparatus.